<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="lib/vue.js"></script>
</head>
<body>
  <div id="app">
    <div id="box">box</div>
    <!-- ref放到标签上表示可以使用DOM操作 -->
    <div ref="content">content</div>
  </div>
</body>

<script>
  new Vue({
    el: '#app',
    mounted () {
      document.getElementById('box').style.color = "red"
      this.$refs.content.style.color = "green"
    },
  })
</script>
</html>